2007 Pearson Education, Inc. All rights reserved. 1 7 7 C Pointers

1

2007 Pearson Education, Inc. All rights reserved.

77C Pointers

2


Addresses are given to us toconceal our whereabouts.

—Saki (H. H. Munro)

By indirection find direction out.—William Shakespeare

Many things, having full referenceTo one consent, may work contrariously.

—William Shakespeare

You will find it a very good practice alwaysto verify your references, sir!

Dr. Routh

3


OBJECTIVES

In this chapter you will learn: Pointers and pointer operators. To use pointers to pass arguments to functions

by reference. The close relationships among pointers, arrays

and strings. To use pointers to functions. To define and use arrays of strings.

4


7.1 Introduction

7.2 Pointer Variable Definitions and Initialization

7.3 Pointer Operators

7.4 Passing Arguments to Functions by Reference

7.5 Using the const Qualifier with Pointers

7.6 Bubble Sort Using Call-by-Reference

7.7 sizeof Operator

7.8 Pointer Expressions and Pointer Arithmetic

7.9 Relationship between Pointers and Arrays

7.10 Arrays of Pointers

7.11 Case Study: Card Shuffling and Dealing Simulation

7.12 Pointers to Functions

5


7.1 Introduction

Pointers – Powerful, but difficult to master

– Simulate call-by-reference

– Close relationship with arrays and strings

6



Pointer variables– Contain memory addresses as their values

– Normal variables contain a specific value (direct reference)

– Pointers contain address of a variable that has a specific value (indirect reference)

– Indirection – referencing a pointer value

7


Fig. 7.1 | Directly and indirectly referencing a variable.

8



Pointer definitions– * used with pointer variables

int *myPtr;

– Defines a pointer to an int (pointer of type int *)

– Multiple pointers require using a * before each variable definition

int *myPtr1, *myPtr2;

– Can define pointers to any data type

– Initialize pointers to 0, NULL, or an address- 0 or NULL – points to nothing (NULL preferred)

9


Common Programming Error 7.1

The asterisk (*) notation used to declare pointer variables does not distribute to all variable names in a declaration. Each pointer must be declared with the * prefixed to the name; e.g., if you wish to declare xPtr and yPtr as int pointers, use int *xPtr, *yPtr;.

10


Good Programming Practice 7.1

Include the letters ptr in pointer variable names to make it clear that these variables are pointers and thus need to be handled appropriately.

11


Error-Prevention Tip 7.1

Initialize pointers to prevent unexpected results.

12



& (address operator)

– Returns address of operandint y = 5;

int *yPtr;

yPtr = &y; /* yPtr gets address of y */

yPtr “points to” y

13


Fig. 7.2 | Graphical representation of a pointer pointing to an integer variable in memory.

14


Fig. 7.3 | Representation of y and yPtr in memory.

15



* (indirection/dereferencing operator)– Returns a synonym/alias of what its operand points to

– *yptr returns y (because yptr points to y)

– * can be used for assignment - Returns alias to an object

*yptr = 7; /* changes y to 7 */

– Dereferenced pointer (operand of *) must be an lvalue (no constants)

* and & are inverses – They cancel each other out

16



Dereferencing a pointer that has not been properly initialized or that has not been assigned to point to a specific location in memory is an error. This could cause a fatal execution-time error, or it could accidentally modify important data and allow the program to run to completion with incorrect results.

17


1 /* Fig. 7.4: fig07_04.c

2 Using the & and * operators */

3 #include <stdio.h>

4

5 int main( void )

6 {

7 int a; /* a is an integer */

8 int *aPtr; /* aPtr is a pointer to an integer */

9

10 a = 7;

11 aPtr = &a; /* aPtr set to address of a */

12

13 printf( "The address of a is %p"

14 "\nThe value of aPtr is %p", &a, aPtr );

15

16 printf( "\n\nThe value of a is %d"

17 "\nThe value of *aPtr is %d", a, *aPtr );

18

19 printf( "\n\nShowing that * and & are complements of "

20 "each other\n&*aPtr = %p"

21 "\n*&aPtr = %p\n", &*aPtr, *&aPtr );

22

23 return 0; /* indicates successful termination */

24

25 } /* end main */

Outline

fig07_04.c

(1 of 2 )

If aPtr points to a, then &a and aPtr have the same value.

a and *aPtr have the same value

&*aPtr and *&aPtr have the same value

18


The address of a is 0012FF7C The value of aPtr is 0012FF7C

The value of a is 7 The value of *aPtr is 7

Showing that * and & are complements of each other. &*aPtr = 0012FF7C

*&aPtr = 0012FF7C

Outline

fig07_04.c

(2 of 2 )

19


Operators Associativity Type

() [ ] left to right highest

+ - ++ - ++ -- ! * & (type) right to left unary

* / left to right multiplicative

+ - left to right additive

< <= > >= left to right relational

== != left to right Equality

&& left to right logical and

|| left to right logical OR

?: right to left conditional

= += -= *= /= %= right to left assignment

, left to right comma

Fig. 7.5 | Operator precedence.

20


7.4 Calling Functions by Reference

Call by reference with pointer arguments– Pass address of argument using & operator– Allows you to change actual location in memory– Arrays are not passed with & because the array name is

already a pointer

* operator – Used as alias/nickname for variable inside of function

void double( int *number ) {

*number = 2 * ( *number ); }

– *number used as nickname for the variable passed

21


1 /* Fig. 7.6: fig07_06.c

2 Cube a variable using call-by-value */


4

5 int cubeByValue( int n ); /* prototype */

6

7 int main( void )

8 {

9 int number = 5; /* initialize number */

10

11 printf( "The original value of number is %d", number );

12

13 /* pass number by value to cubeByValue */

14 number = cubeByValue( number );

15

16 printf( "\nThe new value of number is %d\n", number );

17


19

20 } /* end main */

21

22 /* calculate and return cube of integer argument */

23 int cubeByValue( int n )

24 {

25 return n * n * n; /* cube local variable n and return result */

26

27 } /* end function cubeByValue */ The original value of number is 5 The new value of number is 125

Outline

fig07_06.c

22



Not dereferencing a pointer when it is necessary to do so in order to obtain the value to which the pointer points is a syntax error.

23


1 /* Fig. 7.7: fig07_07.c

2 Cube a variable using call-by-reference with a pointer argument */

3


5

6 void cubeByReference( int *nPtr ); /* prototype */

7

8 int main( void )

9 {

10 int number = 5; /* initialize number */

11

12 printf( "The original value of number is %d", number );

13

14 /* pass address of number to cubeByReference */

15 cubeByReference( &number );

16

17 printf( "\nThe new value of number is %d\n", number );

18


20

21 } /* end main */

22

23 /* calculate cube of *nPtr; modifies variable number in main */

24 void cubeByReference( int *nPtr )

25 {

26 *nPtr = *nPtr * *nPtr * *nPtr; /* cube *nPtr */

27 } /* end function cubeByReference */ The original value of number is 5 The new value of number is 125

Outline

fig07_07.c

Function prototype takes a pointer argument

Function cubeByReference is passed an address, which can be the value of a pointer variable

In this program, *nPtr is number, so this statement modifies the value of number itself.

24


Fig. 7.8 | Analysis of a typical call-by-value.

25


Fig. 7.9 | Analysis of a typical call-by-reference with a pointer argument.

26



Use call-by-value to pass arguments to a function unless the caller explicitly requiresthe called function to modify the value of the argument variable in the caller’s environment. This prevents accidental modification of the caller’s arguments and is another example of the principle of least privilege.

27


7.5 Using the const Qualifier with Pointers

const qualifier– Variable cannot be changed– Use const if function does not need to change a variable– Attempting to change a const variable produces an error

const pointers– Point to a constant memory location– Must be initialized when defined– int *const myPtr = &x;

- Type int *const – constant pointer to an int– const int *myPtr = &x;

- Modifiable pointer to a const int– const int *const Ptr = &x;

- const pointer to a const int- x itself can be changed, but not *Ptr

28


Software Engineering Observation 7.1

The const qualifier can be used to enforce the principle of least privilege. Using the principle of least privilege to properly design software reduces debugging time and improper side effects, making a program easier to modify and maintain.

29


Portability Tip 7.1

Although const is well defined in StandardC, some compilers do not enforce it.

30



If a variable does not (or should not) change in the body of a function to which it is passed, the variable should be declared const to ensure that it is not accidentally modified.

31



Only one value can be altered in a calling function when call-by-value is used. That value must be assigned from the return value of the function. To modify multiple values in a calling function, call-by-reference must be used.

32



Before using a function, check its function prototype to determine if the function is able to modify the values passed to it.

33



Being unaware that a function is expecting pointers as arguments for call-by-reference and passing arguments call-by-value. Some compilers take the values assuming they are pointers and dereference the values as pointers. At runtime, memory-access violations or segmentation faults are often generated. Other compilers catch the mismatch in types between arguments and parameters and generate error messages.

34


1 /* Fig. 7.10: fig07_10.c

2 Converting lowercase letters to uppercase letters

3 using a non-constant pointer to non-constant data */

4


6 #include <ctype.h>

7

8 void convertToUppercase( char *sPtr ); /* prototype */

9

10 int main( void )

11 {

12 char string[] = "characters and $32.98"; /* initialize char array */

13

14 printf( "The string before conversion is: %s", string );

15 convertToUppercase( string );

16 printf( "\nThe string after conversion is: %s\n", string );

17


19

20 } /* end main */

21

Outline

fig07_10.c

(1 of 2 )

Both sPtr and *sPtr are modifiable

35


22 /* convert string to uppercase letters */

23 void convertToUppercase( char *sPtr )

24 {

25 while ( *sPtr != '\0' ) { /* current character is not '\0' */

26

27 if ( islower( *sPtr ) ) { /* if character is lowercase, */

28 *sPtr = toupper( *sPtr ); /* convert to uppercase */

29 } /* end if */

30

31 ++sPtr; /* move sPtr to the next character */

32 } /* end while */

33

34 } /* end function convertToUppercase */ The string before conversion is: characters and $32.98 The string after conversion is: CHARACTERS AND $32.98

Outline

fig07_10.c

(2 of 2 )

Both sPtr and *sPtr are modified by the convertToUppercase function

36


1 /* Fig. 7.11: fig07_11.c

2 Printing a string one character at a time using

3 a non-constant pointer to constant data */

4


6

7 void printCharacters( const char *sPtr );

8

9 int main( void )

10 {

11 /* initialize char array */

12 char string[] = "print characters of a string";

13

14 printf( "The string is:\n" );

15 printCharacters( string );

16 printf( "\n" );

17


19

20 } /* end main */

21

Outline

fig07_11.c

(1 of 2 )

Pointer variable sPtr is modifiable, but the data to which it points, *sPtr, is not

37


22 /* sPtr cannot modify the character to which it points,

23 i.e., sPtr is a "read-only" pointer */

24 void printCharacters( const char *sPtr )

25 {

26 /* loop through entire string */

27 for ( ; *sPtr != '\0'; sPtr++ ) { /* no initialization */

28 printf( "%c", *sPtr );

29 } /* end for */

30

31 } /* end function printCharacters */ The string is: print characters of a string

Outline

fig07_11.c

(2 of 2 )

sPtr is modified by function printCharacters

38


1 /* Fig. 7.12: fig07_12.c

2 Attempting to modify data through a

3 non-constant pointer to constant data. */


5 void f( const int *xPtr ); /* prototype */

6

7

8 int main( void )

9 {

10 int y; /* define y */

11

12 f( &y ); /* f attempts illegal modification */

13


15

16 } /* end main */

17

18 /* xPtr cannot be used to modify the

19 value of the variable to which it points */

20 void f( const int *xPtr )

21 {

22 *xPtr = 100; /* error: cannot modify a const object */

23 } /* end function f */ Compiling... FIG07_12.c

c:\books\2006\chtp5\examples\ch07\fig07_12.c(22) : error C2166: l-value specifies const object Error executing cl.exe.

FIG07_12.exe - 1 error(s), 0 warning(s)

Outline

fig07_12.cPointer variable xPtr is modifiable, but the data

to which it points, *xPtr, is not

*xPtr has the const qualifier, so attempting to modify its value causes an error

39


Performance Tip 7.1

Pass large objects such as structures using pointers to constant data to obtain the performance benefits of call-by-reference and the security of call-by-value.

40


1 /* Fig. 7.13: fig07_13.c

2 Attempting to modify a constant pointer to non-constant data */


4

5 int main( void )

6 {

7 int x; /* define x */


9

10 /* ptr is a constant pointer to an integer that can be modified

11 through ptr, but ptr always points to the same memory location */

12 int * const ptr = &x;

13

14 *ptr = 7; /* allowed: *ptr is not const */

15 ptr = &y; /* error: ptr is const; cannot assign new address */

16


18

19 } /* end main */ Compiling...

FIG07_13.c c:\books\2006\chtp5\Examples\ch07\FIG07_13.c(15) : error C2166: l-value specifies const object

Error executing cl.exe. FIG07_13.exe - 1 error(s), 0 warning(s)

Outline

fig07_13.c

Pointer ptr is not modifiable, but the data to which it points, *ptr, can be changed

41


1 /* Fig. 7.14: fig07_14.c

2 Attempting to modify a constant pointer to constant data. */


4

5 int main( void )

6 {

7 int x = 5; /* initialize x */


9

10 /* ptr is a constant pointer to a constant integer. ptr always

11 points to the same location; the integer at that location

12 cannot be modified */

13 const int *const ptr = &x;

14

15 printf( "%d\n", *ptr );

16

17 *ptr = 7; /* error: *ptr is const; cannot assign new value */

18 ptr = &y; /* error: ptr is const; cannot assign new address */

19


21

22 } /* end main */ Compiling...

FIG07_14.c c:\books\2006\chtp5\Examples\ch07\FIG07_14.c(17) : error C2166: l-value specifies const object

c:\books\2006\chtp5\Examples\ch07\FIG07_14.c(18) : error C2166: l-value specifies const object Error executing cl.exe.

FIG07_12.exe - 2 error(s), 0 warning(s)

Outline

fig07_14.c

Neither pointer sPtr nor the data to which it points, *sPtr, is modifiable

42


7.6 Bubble Sort Using Call-by-Reference

Implement bubble sort using pointers– Swap two elements

– swap function must receive address (using &) of array elements- Array elements have call-by-value default

– Using pointers and the * operator, swap can switch array elements

PsuedocodeInitialize array

print data in original order

Call function bubble sort

print sorted array

Define bubble sort

43


1 /* Fig. 7.15: fig07_15.c

2 This program puts values into an array, sorts the values into

3 ascending order, and prints the resulting array. */


5 #define SIZE 10

6

7 void bubbleSort( int * const array, const int size ); /* prototype */

8

9 int main( void )

10 {

11 /* initialize array a */

12 int a[ SIZE ] = { 2, 6, 4, 8, 10, 12, 89, 68, 45, 37 };

13

14 int i; /* counter */

15

16 printf( "Data items in original order\n" );

17

18 /* loop through array a */

19 for ( i = 0; i < SIZE; i++ ) {

20 printf( "%4d", a[ i ] );

21 } /* end for */

22

23 bubbleSort( a, SIZE ); /* sort the array */

24

25 printf( "\nData items in ascending order\n" );

26

27 /* loop through array a */

28 for ( i = 0; i < SIZE; i++ ) {

29 printf( "%4d", a[ i ] );

30 } /* end for */

Outline

fig07_15.c

(1 of 3 )

44


31

32 printf( "\n" );

33


35

36 } /* end main */

37

38 /* sort an array of integers using bubble sort algorithm */

39 void bubbleSort( int * const array, const int size )

40 {

41 void swap( int *element1Ptr, int *element2Ptr ); /* prototype */

42 int pass; /* pass counter */

43 int j; /* comparison counter */

44

45 /* loop to control passes */

46 for ( pass = 0; pass < size - 1; pass++ ) {

47

48 /* loop to control comparisons during each pass */

49 for ( j = 0; j < size - 1; j++ ) {

50

51 /* swap adjacent elements if they are out of order */

52 if ( array[ j ] > array[ j + 1 ] ) {

53 swap( &array[ j ], &array[ j + 1 ] );

54 } /* end if */

55

56 } /* end inner for */

57

58 } /* end outer for */

59

60 } /* end function bubbleSort */

Outline

fig07_15.c

(2 of 3 )

45


61

62 /* swap values at memory locations to which element1Ptr and

63 element2Ptr point */

64 void swap( int *element1Ptr, int *element2Ptr )

65 {

66 int hold = *element1Ptr;

67 *element1Ptr = *element2Ptr;

68 *element2Ptr = hold;

69 } /* end function swap */ Data items in original order

2 6 4 8 10 12 89 68 45 37 Data items in ascending order 2 4 6 8 10 12 37 45 68 89

Outline

fig07_15.c

(3 of 3 )Function swap changes the values of the ints

that the two pointers point to

46



Placing function prototypes in the definitionsof other functions enforces the principle of least privilege by restricting proper function calls to the functions in which the prototypes appear.

47



When passing an array to a function, also pass the size of the array. This helps make the function reusable in many programs.

48



Global variables usually violate the principle of least privilege and can lead to poor software engineering. Global variables should be used only to represent truly shared resources, such as the time of day.

49


7.7 sizeof Operator

sizeof– Returns size of operand in bytes– For arrays: size of 1 element * number of elements– if sizeof( int ) equals 4 bytes, then

int myArray[ 10 ];printf( "%d", sizeof( myArray ) );

- will print 40

sizeof can be used with– Variable names– Type name– Constant values

50


1 /* Fig. 7.16: fig07_16.c

2 Sizeof operator when used on an array name

3 returns the number of bytes in the array. */


5

6 size_t getSize( float *ptr ); /* prototype */

7

8 int main( void )

9 {

10 float array[ 20 ]; /* create array */

11

12 printf( "The number of bytes in the array is %d"

13 "\nThe number of bytes returned by getSize is %d\n",

14 sizeof( array ), getSize( array ) );

15


17

18 } /* end main */

19

20 /* return size of ptr */

21 size_t getSize( float *ptr )

22 {

23 return sizeof( ptr );

24

25 } /* end function getSize */ The number of bytes in the array is 80 The number of bytes returned by getSize is 4

Outline

fig07_16.c

floats take up 4 bytes in memory, so 20 floats take up 80 bytes

51


Performance Tip 7.2

sizeof is a compile-time operator, so it does not incur any execution-time overhead.

52


Portability Tip 7.2

The number of bytes used to store a particular data type may vary between systems. When writing programs that depend on data type sizes and that will run on several computer systems, use sizeof to determine the number of bytes used to store the data types.

53


1 /* Fig. 7.17: fig07_17.c

2 Demonstrating the sizeof operator */


4

5 int main( void )

6 {

7 char c;

8 short s;

9 int i;

10 long l;

11 float f;

12 double d;

13 long double ld;

14 int array[ 20 ]; /* create array of 20 int elements */

15 int *ptr = array; /* create pointer to array */

16

Outline

fig07_17.c

(1 of 2 )

54


17 printf( " sizeof c = %d\tsizeof(char) = %d"

18 "\n sizeof s = %d\tsizeof(short) = %d"

19 "\n sizeof i = %d\tsizeof(int) = %d"

20 "\n sizeof l = %d\tsizeof(long) = %d"

21 "\n sizeof f = %d\tsizeof(float) = %d"

22 "\n sizeof d = %d\tsizeof(double) = %d"

23 "\n sizeof ld = %d\tsizeof(long double) = %d"

24 "\n sizeof array = %d"

25 "\n sizeof ptr = %d\n",

26 sizeof c, sizeof( char ), sizeof s, sizeof( short ), sizeof i,

27 sizeof( int ), sizeof l, sizeof( long ), sizeof f,

28 sizeof( float ), sizeof d, sizeof( double ), sizeof ld,

29 sizeof( long double ), sizeof array, sizeof ptr );

30


32

33 } /* end main */ sizeof c = 1 sizeof(char) = 1 sizeof s = 2 sizeof(short) = 2 sizeof i = 4 sizeof(int) = 4

sizeof l = 4 sizeof(long) = 4 sizeof f = 4 sizeof(float) = 4 sizeof d = 8 sizeof(double) = 8

sizeof ld = 8 sizeof(long double) = 8 sizeof array = 80 sizeof ptr = 4

Outline

fig07_17.c

(2 of 2 )

55



Arithmetic operations can be performed on pointers

– Increment/decrement pointer (++ or --)

– Add an integer to a pointer( + or += , - or -=)

– Pointers may be subtracted from each other

– Operations meaningless unless performed on an array

56



5 element int array on machine with 4 byte ints– vPtr points to first element v[ 0 ]

- at location 3000 (vPtr = 3000)

– vPtr += 2; sets vPtr to 3008- vPtr points to v[ 2 ] (incremented by 2), but the machine

has 4 byte ints, so it points to address 3008

57


Fig. 7.18 | Array v and a pointer variable vPtr that points to v.

58


Fig. 7.19 | The pointer vPtr after pointer arithmetic.

59


Portability Tip 7.3

Most computers today have 2-byte or 4-byte integers. Some of the newer machines use 8-byte integers. Because the results of pointer arithmetic depend on the size of the objects a pointer points to, pointer arithmetic is machine dependent.

60



Subtracting pointers– Returns number of elements from one to the other. If

vPtr2 = v[ 2 ];

vPtr = v[ 0 ];

– vPtr2 - vPtr would produce 2

Pointer comparison ( <, == , > )– See which pointer points to the higher numbered array

element

– Also, see if a pointer points to 0

61



Using pointer arithmetic on a pointer that does not refer to an element in an array.

62



Subtracting or comparing two pointers that do not refer to elements in the same array.

63



Running off either end of an array when using pointer arithmetic.

64



Pointers of the same type can be assigned to each other

– If not the same type, a cast operator must be used

– Exception: pointer to void (type void *)- Generic pointer, represents any type

- No casting needed to convert a pointer to void pointer

- void pointers cannot be dereferenced

65



Assigning a pointer of one type to a pointer of another type if neither is of type void * is a syntax error.

66



Dereferencing a void * pointer is a syntax error.

67


7.9 Relationship Between Pointers and Arrays

Arrays and pointers closely related– Array name like a constant pointer

– Pointers can do array subscripting operations

Define an array b[ 5 ] and a pointer bPtr– To set them equal to one another use:

bPtr = b;

- The array name (b) is actually the address of first element of the array b[ 5 ]

bPtr = &b[ 0 ]

- Explicitly assigns bPtr to address of first element of b

68


7.9 Relationship Between Pointers and Arrays

– Element b[ 3 ] - Can be accessed by *( bPtr + 3 )

Where 3 is called the offset. Called pointer/offset notation

- Can be accessed by bptr[ 3 ]

Called pointer/subscript notation

bPtr[ 3 ] same as b[ 3 ]

- Can be accessed by performing pointer arithmetic on the array itself

*( b + 3 )

69



Attempting to modify an array name with pointer arithmetic is a syntax error.

70


1 /* Fig. 7.20: fig07_20.cpp

2 Using subscripting and pointer notations with arrays */

3


5

6 int main( void )

7 {

8 int b[] = { 10, 20, 30, 40 }; /* initialize array b */

9 int *bPtr = b; /* set bPtr to point to array b */


11 int offset; /* counter */

12

13 /* output array b using array subscript notation */

14 printf( "Array b printed with:\nArray subscript notation\n" );

15

16 /* loop through array b */

17 for ( i = 0; i < 4; i++ ) {

18 printf( "b[ %d ] = %d\n", i, b[ i ] );

19 } /* end for */

20

21 /* output array b using array name and pointer/offset notation */

22 printf( "\nPointer/offset notation where\n"

23 "the pointer is the array name\n" );

24


26 for ( offset = 0; offset < 4; offset++ ) {

27 printf( "*( b + %d ) = %d\n", offset, *( b + offset ) );

28 } /* end for */

29

Outline

fig07_20.c

(1 of 3 )

Array subscript notation

Pointer/offset notation

71


30 /* output array b using bPtr and array subscript notation */

31 printf( "\nPointer subscript notation\n" );

32


34 for ( i = 0; i < 4; i++ ) {

35 printf( "bPtr[ %d ] = %d\n", i, bPtr[ i ] );

36 } /* end for */

37

38 /* output array b using bPtr and pointer/offset notation */

39 printf( "\nPointer/offset notation\n" );

40


42 for ( offset = 0; offset < 4; offset++ ) {

43 printf( "*( bPtr + %d ) = %d\n", offset, *( bPtr + offset ) );

44 } /* end for */

45


47

48 } /* end main */ Array b printed with: Array subscript notation b[ 0 ] = 10 b[ 1 ] = 20 b[ 2 ] = 30 b[ 3 ] = 40 (continued on next slide… )

Outline

fig07_20.c

(2 of 3 )

Pointer subscript notation

Pointer offset notation

72


(continued from previous slide…) Pointer/offset notation where the pointer is the array name *( b + 0 ) = 10 *( b + 1 ) = 20 *( b + 2 ) = 30 *( b + 3 ) = 40 Pointer subscript notation bPtr[ 0 ] = 10 bPtr[ 1 ] = 20 bPtr[ 2 ] = 30 bPtr[ 3 ] = 40 Pointer/offset notation *( bPtr + 0 ) = 10 *( bPtr + 1 ) = 20 *( bPtr + 2 ) = 30 *( bPtr + 3 ) = 40

Outline

fig07_20.c

(3 of 3 )

73


1 /* Fig. 7.21: fig07_21.c

2 Copying a string using array notation and pointer notation. */


4

5 void copy1( char * const s1, const char * const s2 ); /* prototype */

6 void copy2( char *s1, const char *s2 ); /* prototype */

7

8 int main( void )

9 {

10 char string1[ 10 ]; /* create array string1 */

11 char *string2 = "Hello"; /* create a pointer to a string */

12 char string3[ 10 ]; /* create array string3 */

13 char string4[] = "Good Bye"; /* create a pointer to a string */

14

15 copy1( string1, string2 );

16 printf( "string1 = %s\n", string1 );

17

18 copy2( string3, string4 );

19 printf( "string3 = %s\n", string3 );

20


22

23 } /* end main */

24

Outline

fig07_21.c

(1 of 2 )

74


25 /* copy s2 to s1 using array notation */

26 void copy1( char * const s1, const char * const s2 )

27 {


29

30 /* loop through strings */

31 for ( i = 0; ( s1[ i ] = s2[ i ] ) != '\0'; i++ ) {

32 ; /* do nothing in body */

33 } /* end for */

34

35 } /* end function copy1 */

36

37 /* copy s2 to s1 using pointer notation */

38 void copy2( char *s1, const char *s2 )

39 {

40 /* loop through strings */

41 for ( ; ( *s1 = *s2 ) != '\0'; s1++, s2++ ) {

42 ; /* do nothing in body */

43 } /* end for */

44

45 } /* end function copy2 */ string1 = Hello

string3 = Good Bye

Outline

fig07_21.c

(2 of 2 )

Condition of for loop actually performs an action

75


7.10 Arrays of Pointers

Arrays can contain pointers For example: an array of strings

char *suit[ 4 ] = { "Hearts", "Diamonds", "Clubs", "Spades" };

– Strings are pointers to the first character

– char * – each element of suit is a pointer to a char

– The strings are not actually stored in the array suit, only pointers to the strings are stored

76


Fig. 7.22 | Graphical representation of the suit array.

77



Card shuffling program– Use array of pointers to strings

– Use double subscripted array (suit, face)

– The numbers 1-52 go into the array- Representing the order in which the cards are dealt

78


Performance Tip 7.3

Sometimes an algorithm that emerges in a “natural” way can contain subtle performance problems, such as indefinite postponement. Seek algorithms that avoid indefinite postponement.

79


Fig. 7.23 | Double-subscripted array representation of a deck of cards.

80



Pseudocode– Top level:

Shuffle and deal 52 cards

– First refinement:

Initialize the suit array

Initialize the face array

Initialize the deck array

Shuffle the deck

Deal 52 cards

81



– Second refinement- Convert shuffle the deck to

For each of the 52 cardsPlace card number in randomly selected unoccupied slot of deck

- Convert deal 52 cards to

For each of the 52 cardsFind card number in deck array and print face and suit of card

82



– Third refinement- Convert shuffle the deck to

Choose slot of deck randomly

While chosen slot of deck has been previously chosenChoose slot of deck randomly

Place card number in chosen slot of deck

- Convert deal 52 cards to

For each slot of the deck arrayIf slot contains card number Print the face and suit of the card

83


1 /* Fig. 7.24: fig07_24.c

2 Card shuffling dealing program */


4 #include <stdlib.h>

5 #include <time.h>

6

7 /* prototypes */

8 void shuffle( int wDeck[][ 13 ] );

9 void deal( const int wDeck[][ 13 ], const char *wFace[],

10 const char *wSuit[] );

11

12 int main( void )

13 {

14 /* initialize suit array */

15 const char *suit[ 4 ] = { "Hearts", "Diamonds", "Clubs", "Spades" };

16

17 /* initialize face array */

18 const char *face[ 13 ] =

19 { "Ace", "Deuce", "Three", "Four",

20 "Five", "Six", "Seven", "Eight",

21 "Nine", "Ten", "Jack", "Queen", "King" };

22

Outline

fig07_24.c

(1 of 4 )

suit and face arrays are arrays of pointers

84


23 /* initialize deck array */

24 int deck[ 4 ][ 13 ] = { 0 };

25

26 srand( time( 0 ) ); /* seed random-number generator */

27

28 shuffle( deck );

29 deal( deck, face, suit );

30


32

33 } /* end main */

34

35 /* shuffle cards in deck */

36 void shuffle( int wDeck[][ 13 ] )

37 {

38 int row; /* row number */

39 int column; /* column number */

40 int card; /* counter */

41

42 /* for each of the 52 cards, choose slot of deck randomly */

43 for ( card = 1; card <= 52; card++ ) {

44

45 /* choose new random location until unoccupied slot found */

46 do {

47 row = rand() % 4;

48 column = rand() % 13;

49 } while( wDeck[ row ][ column ] != 0 ); /* end do...while */

50

Outline

fig07_24.c

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do…while loop selects a random spot for each card

85


51 /* place card number in chosen slot of deck */

52 wDeck[ row ][ column ] = card;

53 } /* end for */

54

55 } /* end function shuffle */

56

57 /* deal cards in deck */

58 void deal( const int wDeck[][ 13 ], const char *wFace[],

59 const char *wSuit[] )

60 {

61 int card; /* card counter */

62 int row; /* row counter */

63 int column; /* column counter */

64

65 /* deal each of the 52 cards */

66 for ( card = 1; card <= 52; card++ ) {

67 /* loop through rows of wDeck */

68

69 for ( row = 0; row <= 3; row++ ) {

70

71 /* loop through columns of wDeck for current row */

72 for ( column = 0; column <= 12; column++ ) {

Outline

fig07_24.c

(3 of 4 )

86


73

74 /* if slot contains current card, display card */

75 if ( wDeck[ row ][ column ] == card ) {

76 printf( "%5s of %-8s%c", wFace[ column ], wSuit[ row ],

77 card % 2 == 0 ? '\n' : '\t' );

78 } /* end if */

79

80 } /* end for */

81

82 } /* end for */

83

84 } /* end for */

85

86 } /* end function deal */

Outline

fig07_24.c

(4 of 4 )

87


Nine of Hearts Five of Clubs Queen of Spades Three of Spades Queen of Hearts Ace of Clubs King of Hearts Six of Spades Jack of Diamonds Five of Spades Seven of Hearts King of Clubs Three of Clubs Eight of Hearts Three of Diamonds Four of Diamonds Queen of Diamonds Five of Diamonds Six of Diamonds Five of Hearts Ace of Spades Six of Hearts Nine of Diamonds Queen of Clubs Eight of Spades Nine of Clubs Deuce of Clubs Six of Clubs Deuce of Spades Jack of Clubs Four of Clubs Eight of Clubs Four of Spades Seven of Spades Seven of Diamonds Seven of Clubs King of Spades Ten of Diamonds Jack of Hearts Ace of Hearts Jack of Spades Ten of Clubs Eight of Diamonds Deuce of Diamonds Ace of Diamonds Nine of Spades Four of Hearts Deuce of Hearts King of Diamonds Ten of Spades Three of Hearts Ten of Hearts

Outline

88



Pointer to function– Contains address of function

– Similar to how array name is address of first element

– Function name is starting address of code that defines function

Function pointers can be – Passed to functions

– Stored in arrays

– Assigned to other function pointers

89



Example: bubblesort– Function bubble takes a function pointer

- bubble calls this helper function

- this determines ascending or descending sorting

– The argument in bubblesort for the function pointer:int ( *compare )( int a, int b )

tells bubblesort to expect a pointer to a function that takes two ints and returns an int

– If the parentheses were left out:int *compare( int a, int b )

- Defines a function that receives two integers and returns a pointer to a int

90


1 /* Fig. 7.26: fig07_26.c

2 Multipurpose sorting program using function pointers */


4 #define SIZE 10

5

6 /* prototypes */

7 void bubble( int work[], const int size, int (*compare)( int a, int b ) );

8 int ascending( int a, int b );

9 int descending( int a, int b );

10

11 int main( void )

12 {

13 int order; /* 1 for ascending order or 2 for descending order */

14 int counter; /* counter */

15

16 /* initialize array a */

17 int a[ SIZE ] = { 2, 6, 4, 8, 10, 12, 89, 68, 45, 37 };

18

19 printf( "Enter 1 to sort in ascending order,\n"

20 "Enter 2 to sort in descending order: " );

21 scanf( "%d", &order );

22

23 printf( "\nData items in original order\n" );

24

25 /* output original array */

26 for ( counter = 0; counter < SIZE; counter++ ) {

27 printf( "%5d", a[ counter ] );

28 } /* end for */

29

Outline

fig07_26.c

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bubble function takes a function pointer as an argument

91


30 /* sort array in ascending order; pass function ascending as an

31 argument to specify ascending sorting order */

32 if ( order == 1 ) {

33 bubble( a, SIZE, ascending );

34 printf( "\nData items in ascending order\n" );

35 } /* end if */

36 else { /* pass function descending */

37 bubble( a, SIZE, descending );

38 printf( "\nData items in descending order\n" );

39 } /* end else */

40

41 /* output sorted array */

42 for ( counter = 0; counter < SIZE; counter++ ) {

43 printf( "%5d", a[ counter ] );

44 } /* end for */

45

46 printf( "\n" );

47


49

50 } /* end main */

51

Outline

fig07_26.c

(2 of 4 )

depending on the user’s choice, the bubble function uses either the ascending or descending function to sort the array

92


52 /* multipurpose bubble sort; parameter compare is a pointer to

53 the comparison function that determines sorting order */

54 void bubble( int work[], const int size, int (*compare)( int a, int b ) )

55 {

56 int pass; /* pass counter */

57 int count; /* comparison counter */

58

59 void swap( int *element1Ptr, int *element2ptr ); /* prototype */

60

61 /* loop to control passes */

62 for ( pass = 1; pass < size; pass++ ) {

63

64 /* loop to control number of comparisons per pass */

65 for ( count = 0; count < size - 1; count++ ) {

66

67 /* if adjacent elements are out of order, swap them */

68 if ( (*compare)( work[ count ], work[ count + 1 ] ) ) {

69 swap( &work[ count ], &work[ count + 1 ] );

70 } /* end if */

71

72 } /* end for */

73

74 } /* end for */

75

76 } /* end function bubble */

77

Outline

fig07_26.c

(3 of 4 )

Note that what the program considers “out of order” is dependent on the function pointer that was passed to the bubble function

93


78 /* swap values at memory locations to which element1Ptr and

79 element2Ptr point */

80 void swap( int *element1Ptr, int *element2Ptr )

81 {

82 int hold; /* temporary holding variable */

83

84 hold = *element1Ptr;

85 *element1Ptr = *element2Ptr;

86 *element2Ptr = hold;

87 } /* end function swap */

88

89 /* determine whether elements are out of order for an ascending

90 order sort */

91 int ascending( int a, int b )

92 {

93 return b < a; /* swap if b is less than a */

94

95 } /* end function ascending */

96

97 /* determine whether elements are out of order for a descending

98 order sort */

99 int descending( int a, int b )

100 {

101 return b > a; /* swap if b is greater than a */

102

103 } /* end function descending */

Outline

fig07_26.c

(4 of 4 )

Passing the bubble function ascending will point the program here

Passing the bubble function descending will point the program here

94


Enter 1 to sort in ascending order, Enter 2 to sort in descending order: 1 Data items in original order 2 6 4 8 10 12 89 68 45 37 Data items in ascending order 2 4 6 8 10 12 37 45 68 89 Enter 1 to sort in ascending order, Enter 2 to sort in descending order: 2 Data items in original order 2 6 4 8 10 12 89 68 45 37 Data items in descending order 89 68 45 37 12 10 8 6 4 2

Outline

95


1 /* Fig. 7.28: fig07_28.c

2 Demonstrating an array of pointers to functions */


4

5 /* prototypes */

6 void function1( int a );

7 void function2( int b );

8 void function3( int c );

9

10 int main( void )

11 {

12 /* initialize array of 3 pointers to functions that each take an

13 int argument and return void */

14 void (*f[ 3 ])( int ) = { function1, function2, function3 };

15

16 int choice; /* variable to hold user's choice */

17

18 printf( "Enter a number between 0 and 2, 3 to end: " );

19 scanf( "%d", &choice );

20

Outline

fig07_28.c

(1 of 3 )

Array of pointers to functions

96


21 /* process user's choice */

22 while ( choice >= 0 && choice < 3 ) {

23

24 /* invoke function at location choice in array f and pass

25 choice as an argument */

26 (*f[ choice ])( choice );

27

28 printf( "Enter a number between 0 and 2, 3 to end: ");

29 scanf( "%d", &choice );

30 } /* end while */

31

32 printf( "Program execution completed.\n" );

33


35

36 } /* end main */

37

38 void function1( int a )

39 {

40 printf( "You entered %d so function1 was called\n\n", a );

41 } /* end function1 */

42

43 void function2( int b )

44 {

45 printf( "You entered %d so function2 was called\n\n", b );

46 } /* end function2 */

Outline

fig07_28.c

(2 of 3 )

Function called is dependent on user’s choice

97


47

48 void function3( int c )

49 {

50 printf( "You entered %d so function3 was called\n\n", c );

51 } /* end function3 */ Enter a number between 0 and 2, 3 to end: 0 You entered 0 so function1 was called

Enter a number between 0 and 2, 3 to end: 1 You entered 1 so function2 was called

Enter a number between 0 and 2, 3 to end: 2 You entered 2 so function3 was called

Enter a number between 0 and 2, 3 to end: 3 Program execution completed.

Outline

fig07_28.c

(3 of 3 )

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